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Serfas' Thunderbolt Taillight delivers 35 lumens in an extremely versatile, USB-rechargeable package. Attach the water-resistant silicone body nearly anywhere on your bike, and use the high, low, or two flash modes to make sure cars or cyclists can see you when approaching from behind after dark. At just 50 grams, you won't even notice it's on your bike — but others will!

Seriously, do not stare directly at this thing. If passerbys cannot see you, then they are probably illegal to drive. This thing has four modes, high steady, low steady, high flash, and low flash. I have yet to use this on high at all; it's so bright it's just unnecessary.
The battery life is pretty decent, I run it on low-blinking mode at get at least 8h on a charge. It charges pretty fast too, within a couple hours max.